1. Field of the Invention
The present invention relates to acoustic transducers, including those used in flow meters.
2. Background Art
Transmitting pulses of acoustic energy through a fluid is useful for measuring the state and properties of the fluid, specifically the velocity and temperature. Piezoceramic elements are commonly used in acoustic transducers to generate ultrasonic acoustic pulses or continuous wave fields. However, these ceramics lose polarization if exposed to temperatures in excess of half of their Curie point. For commercially available ceramics, this limits the operating temperature of the ceramic to under 200° C. To operate in fluids above this temperature, one method is to provide a buffer or delay line between the piezoceramic element and the fluid (for example, exhaust gas) as shown in FIG. 1. FIG. 1 depicts an acoustic transducer 10. Transducer 10 includes piezoceramic element 12 and buffer 14 extending through wall 16 into the fluid which is illustrated as exhaust gas. Thermal energy is dissipated at internal convective boundary layer 18, in buffer 14, and at external convective boundary layer 20 as heat conducts upward in buffer 14 toward piezoceramic element 12. Buffers operate by the principle of Fourier's Law of heat conduction:q″=−κ∇T,Where q″ is the heat flux, κ is the thermal conductivity of the material and T is temperature. Detailed solutions of this equation require numerical methods but with some simplifying assumptions, a buffer system can be reduced to a lumped parameter model that can be represented as the equivalent circuit shown in FIG. 2. FIG. 2 illustrates the exhaust, buffer tip, crystal and ambient temperatures, and the thermal resistences of the external convective boundary layer, buffer, and internal convective boundary layer in a lumped parameter model.
For the lumped parameter model illustrated in FIG. 2, the temperature of the crystal is:
      T    crystal    =            T      exhaust        -                  (                              T            exhaust                    -                      T            ambient                          )            ⁢                                    (                                          R                IBL                            +                              R                buffer                                      )                                              R              IBL                        +                          R              buffer                        +                          R              EBL                                      .            
A disadvantage associated with existing buffer systems is that a short buffer has problems when operating with hot fluids, while making the buffer longer requires that the buffer guide the wave front in the desired direction. However, solid buffers fail to effectively guide the acoustic pulse resulting in a dispersive buffer that distorts the ultrasonic pulse and limits the usefulness of the flow meter. A buffer with a solid core and solid cladding created by doping (for example, spray deposition or powder metallurgy) has been proposed. However, the doping techniques used to create the cladding have some disadvantages.
Additional background information may be found in U.S. Pat. Nos. 5,756,360; 4,336,719; 5,217,018; 5,159,838; 6,343,511; 5,241,287; 4,743,870; 5,438,999; 4,297,607; and 6,307,302.
For the foregoing reasons, there is a need for an improved acoustic transducer.